Rails test upload with paperclip-dropbox

I am using paperclip-dropbox gem to store my assets. I want to create a test to ensure assets are properly destroyed if the destroy checkbox is checked. I can see on dropbox the image is uploaded and deleted but just after it's created again. Don't understand why.

Here is my test file:

require 'test_helper'

#
# == Admin namespace
#
module Admin
  #
  # == SettingsController test
  #
  class SettingsControllerTest < ActionController::TestCase
    include Devise::TestHelpers

    setup :initialize_test

    #
    # == Avatar
    #
    test 'should be able to upload logo' do
      upload_dropbox_paperclip_attachment
      setting = assigns(:setting)
      assert setting.logo?
      assert_equal 'bart.png', setting.logo_file_name
      assert_equal 'image/png', setting.logo_content_type
    end

    test 'should be able to destroy logo' do
      upload_dropbox_paperclip_attachment
      remove_dropbox_paperclip_attachment
    end

    private

    def initialize_test
      @setting = settings(:one)
      @administrator = users(:bob)
      sign_in @administrator
    end

    def upload_dropbox_paperclip_attachment
      puts '=== Uploading logo to Dropbox'
      attachment = fixture_file_upload 'images/bart.png', 'image/png'
      patch :update, id: @setting, setting: { logo: attachment }
    end

    def remove_dropbox_paperclip_attachment
      puts '=== Removing logo from Dropbox'
      patch :update, id: @setting, setting: { logo: nil, delete_logo: '1' }
      assert_not assigns(:setting).logo?
    end
  end
end

Do someone know what is wrong with this test ? Thanks

My project:

• Rails 4.2.3
• Ruby 2.2.2